A Text-book of Assaying: For the Use of Those Connected with Mines Part 26

The price of 100 tons of pyrites, containing 2-1/4 per cent. of copper by dry a.s.say, would be got on this standard as follows:--The parcel of ore would contain 2-1/4 tons of copper. This multiplied by the standard gives 160 16s. 0d. From this must be deducted the returning charge, which for 1 ton of ore of this produce would be (12s. 2d.) + (2-1/4(3s.

9d.)) or 1 0s. 7d., and on the 100 tons is 102 18s. 4d. This would leave 57 17s. 10d. as the price of the parcel, or 11s. 7d. per ton.

This would be on the standard returning charge of 45s. (for 8-3/4 per cent. ore); if a smaller returning charge was agreed on, say 38s., the difference in this case, 7s., would be added to the price per ton.

WET METHODS.

The solubility of the ores of copper in acid has already been described, but certain furnace products, such as slags, are best opened up by fusion with fusion mixture and a little nitre.

The method of dissolving varies with the nature of the ore. With 5 grams of pyrites, a single evaporation with 20 c.c. of nitric acid will give a residue completely soluble in 30 c.c. of hydrochloric acid. If the ore carries oxide of iron or similar bodies, these are first dissolved up by boiling with 20 c.c. of hydrochloric acid, and the residue attacked by an addition of 5 c.c. of nitric. When silicates decomposable by acid are present, the solution is evaporated to dryness to render the silica insoluble; the residue extracted with 30 c.c. of hydrochloric acid, and diluted with water to 150 c.c. It is advisable to have the copper in solution as chloride. To separate the copper, heat the solution nearly to boiling (best in a pint flask), and pa.s.s a rapid current of sulphuretted hydrogen for four or five minutes until the precipitate settles readily and the liquid smells of the gas. When iron is present it will be reduced to the ferrous state before the copper sulphide begins to separate. The copper appears as a brown coloration or black precipitate according to the quant.i.ty present. Filter through a coa.r.s.e filter, wash with hot water containing sulphuretted hydrogen, if necessary. Wash the precipitate back into the flask, boil with 10 c.c.

of nitric acid, add soda till alkaline, and pa.s.s sulphuretted hydrogen again. Warm and filter, wash and redissolve in nitric acid, neutralise with ammonia, add ammonic carbonate, boil and filter. The copper freed from impurities will be in the solution. Acidulate and reprecipitate with sulphuretted hydrogen. When the nature of the impurities will allow it, this process may be shortened to first filtering off the gangue, then precipitating with sulphuretted hydrogen and was.h.i.+ng the precipitate on the filter first with water and then with ammonium sulphide.

Having separated the copper as sulphide, its weight is determined as follows. Dry and transfer to a weighed porcelain crucible, mix with a little pure sulphur, and ignite at a red heat for 5 or 10 minutes in a current of hydrogen. Allow to cool while the hydrogen is still pa.s.sing.

Weigh. The subsulphide of copper thus obtained contains 79.85 per cent.

of copper; it is a greyish-black crystalline ma.s.s, which loses no weight on ignition if air is excluded.

Copper may be separated from its solutions by means of sodium hyposulphite. The solution is freed from hydrochloric and nitric acids by evaporation with sulphuric acid; diluted to about a quarter of a litre; heated nearly to boiling; and treated with a hot solution of sodium hyposulphite (added a little at a time) until the precipitate settles and leaves the solution free from colour. The solution contains suspended sulphur. The precipitate is easily washed, and under the proper conditions the separation is complete, but the separation with sulphuretted hydrogen is more satisfactory, since the conditions as to acidity, &c., need not be so exact.

Zinc or iron is sometimes used for separating copper from its solutions, but they are not to be recommended.

ELECTROLYTIC a.s.sAY.

The separation of copper by means of a current of electricity is largely made use of, and forms the basis of the most satisfactory method for the determination of this metal. If the wire closing an electric circuit be broken, and the two ends immersed in a beaker of acidulated water or solution of any salt, the electricity will pa.s.s through the liquid, bringing about some remarkable changes. Hydrogen and the metals will be liberated around that part of the wire connected with the zinc end of the battery, and oxygen, chlorine, and the acid radicals will be set free around the other. Different metals are deposited in this way with varying degrees of ease, and whether or not any particular metal will be deposited depends--(1) on the conditions of the solution as regards acid and other substances present, and (2) on the _intensity_ of the current of electricity used. For a.n.a.lytical purposes the metal should be deposited not only free from the other metals present, but also as a firm coherent film, which may afterwards be manipulated without fear of loss. This is, in the case of copper and many other metals, effected by a simple control of the conditions. It is necessary that the electrodes, or wires which bring the electricity into the solution, should be made of a material to which the deposited metal will adhere, and which will not be attacked by substances originally present or set free in the solution. They are generally made of platinum. There are various arrangements of apparatus used for this purpose, but the following plan and method of working is simple and effective, and has been in daily use with very satisfactory results for the last five or six years.

The battery used is made up of two Daniell cells, coupled up for intensity as shown in fig. 49--that is, with the copper of one connected with the zinc of the other. For eight or ten a.s.says daily the quart size should be used, but for four or five two pint cells will be sufficient.

[Ill.u.s.tration: FIG. 49.]

The outer pot of each cell is made of sheet copper, and must be clean and free from solder on the inside. It is provided near the top with a perforated copper shelf in the shape of a ring, into which the inner or porous cell loosely fits. It is charged with a saturated solution of copper sulphate, and crystals of this salt must be added, and always kept in excess. When the battery is at work copper is being deposited on the inner surface of this pot.

The inner or porous pot contains the zinc rod, and is charged with a dilute acid, made by diluting one volume of sulphuric acid up to ten with water. The object of the porous pot is to prevent the mixing of the acid and copper sulphate solutions, without interrupting the flow of electricity. The copper sulphate solution will last for months, but the acid must be emptied out and recharged daily.

The zinc rods must be well amalgamated by rubbing with mercury under dilute acid until they show a uniformly bright surface. They should not produce a brisk effervescence when placed in the acid in the porous pot before coupling up.

The battery when working is apt to become dirty from the "creeping" of the copper and zinc sulphate solution. It must be kept away from the working bench, and is best kept in a box on the floor.

[Ill.u.s.tration: FIG. 50.]

The connection of the battery with, and the fixing of, the electrodes may be made by any suitable arrangement, but the following is a very convenient plan. The wire from the zinc is connected by means of a binding screw with a piece of stout copper wire, which, at a distance sufficiently great to allow of easy coupling with the battery, is led along the back of a piece of hard wood. This is fixed horizontally about one foot above the working bench. The general arrangement is shown in fig. 50, in which, however, for the sake of economy of s.p.a.ce, the battery is placed on the working bench instead of on the floor. The piece of wood is one inch square and three or four feet long. It is perforated from front to back at distances of six inches by a number of small holes, in which are inserted screws like that shown in fig. 51.

These are known as "terminals," and may be obtained of any electrician.

The head of each screw is soldered to the wire mentioned above as running along the back and as being connected with the zinc end of the battery. These terminals serve to fix the electrodes on which the copper is to be deposited. The wire from the copper end of the battery is similarly connected by a connecting screw (fig. 52) with another wire (H in fig. 53), which runs along the top of the rod and has soldered to it, at distances of six inches, cylindrical spirals of copper wire. These should project from the rod at points about half-way between the terminals already described. They may be made by wrapping copper wire around a black-lead pencil for a length of about three inches.

[Ill.u.s.tration: FIG. 51.]

[Ill.u.s.tration: FIG. 52.]

[Ill.u.s.tration: FIG. 53.]

The rod is perforated from top to bottom with a series of small holes, one in advance of each terminal but as near it as possible. Into these short pieces of gla.s.s tube are inserted to ensure insulation. These receive the other electrodes, which are connected with the wire leading to the copper end of the battery, through the spirals, with the help of a binding screw. The figure will make this clear. (Fig. 53.)

[Ill.u.s.tration: FIG. 54]

~The electrodes~ consist of a platinum spiral and cylinder. The spiral should have the shape shown in A, fig. 54. When in work it is pa.s.sed through one of the holes fitted with gla.s.s tubes and connected with the copper end of the battery. The thickness of the wire of which it is made is unimportant, provided it is stout enough to keep its form and does not easily bend. The spiral will weigh about 8 grams. The cylinder (C, fig. 54) will weigh about 12 grams. It should have the shape shown in the figure. In working it is clamped to one of the terminals, and on it the copper is deposited. A cylinder will serve for the deposition of from 1 to 1.5 gram of copper. It is made by rivetting a square piece of foil on to a stiff piece of wire, and then bending into shape over a gla.s.s tube or piece of rounded wood. Each cylinder carries a distinctive number, and is marked by impressing Roman numerals on the foil with the blade of a knife. The weight of each is carefully taken and recorded.

They lose slightly in weight when in use, but the loss is uniform, and averages half a milligram per month when in daily use. The cylinders are cleaned from deposited copper by dissolving off with nitric acid and was.h.i.+ng with water; and from grease by igniting.

The ~beakers~, to contain the solution of copper to be electrolysed, are ordinary tall beakers of about 200 c.c. capacity, and are marked off at 100 c.c. and 150 c.c. They are supported on movable stands, consisting of wooden blocks about six inches high and three inches across. The bar of wood which carries the connecting wires and electrodes is permanently fixed over the working bench, at such a height that, with the beakers resting on these blocks, the electrodes shall be in position for working.

To fix the electrodes to the rod, remove the stand and beaker and pa.s.s the long limb of the spiral up through one of the gla.s.s tubes. Connect it with the free end of the copper spiral by means of a connecting screw (fig. 52), and then draw out and bend the copper spiral so that the platinum one may hang freely. Screw the wire of the cylinder to the terminal, and, if necessary, bend it so that the cylinder itself may be brought to encircle the rod of the spiral in the manner shown in fig.

53.

The ~general method of working~ is as follows:--The quant.i.ty of ore to be taken for an a.s.say varies with the richness of the ore, as is shown in the following table:--

Percentage of Copper Quant.i.ty of Ore in the Ore. to be taken.

1 to 5 5 grams 5 to 10 3 "

10 to 30 2 "

30 to 50 1.5 "

50 to 100 1 "

The weighed quant.i.ty of ore is dissolved by evaporating with nitric acid and taking up with hydrochloric, as already described. Any coloured residue which may be left is generally organic matter: it is filtered off, calcined, and any copper it contains is estimated colorimetrically.

Nearly always, however, the residue is white and sandy. The copper is separated from the solution as sulphide by means of a rapid current of sulphuretted hydrogen. The liquid is decanted off through a filter, the precipitate washed once with hot water and then rinsed back into the flask (the filter paper being opened out) with a jet of water from a wash bottle. Fifteen c.c. of nitric acid are added to the contents of the flask, which are then briskly boiled until the bulk is reduced to less than 10 c.c. The boiling down is carried out in a cupboard free from cold draughts, so as to prevent the condensation of acid and steam in the neck of the flask. Twenty c.c. of water are next added, and the solution is warmed, and filtered into one of the beakers for electrolysis. The filtrate and was.h.i.+ngs are diluted with water to the 100 c.c. mark, and the solution is then ready for the battery. It must not contain more than 10 per cent. by volume of nitric acid.

The number and weight of the platinum cylinder having been recorded, both electrodes are fixed in position and the wooden block removed from under them. The beaker containing the copper solution is then brought up into its place with one hand, and the block replaced with the other so as to support it. All the a.s.says having been got into position, the connecting wires are joined to the battery. If everything is right bubbles of oxygen at once stream off from the spiral, and the cylinder becomes tarnished by a deposit of copper. If the oxygen comes off but no copper is deposited, it is because the a.s.say solution contains too much nitric acid. If no action whatever takes place, it is because the current is not pa.s.sing. In this case examine the connections to see that they are clean and secure, and the connecting wires to see that they are not touching each other.

The action is allowed to go on for sixteen or seventeen hours, so that it is best to let the current act overnight. In the morning the solutions will appear colourless, and a slow stream of oxygen will still be coming off from the spiral.

A wash-bottle with cold distilled water and two beakers, one with distilled water and the other with alcohol, are got ready. The block is then removed, the spiral loosened and lowered with the beaker. The cylinder is next detached and washed with a stream of water from the wash-bottle, the was.h.i.+ngs being added to the original solution. The current from the battery is not stopped until all the cylinders are washed. After being dipped in the beaker of water and once or twice in that with the alcohol, it is dried in the water-oven for about three minutes, and then weighed. The increase in weight is due to deposited copper. This should be salmon-red in colour, satin-like or crystalline in appearance, and in an even coherent deposit, not removed by rubbing.

It is permanent in air when dry, but sulphuretted hydrogen quickly tarnishes it, producing coloured films. With ores containing even very small proportions of bis.m.u.th, the deposited copper has a dark grey colour, and when much of this metal is present the copper is coated with a grey s.h.a.ggy deposit.

It still remains to determine any copper left undeposited in the solution. This does not generally exceed four or five milligrams, and is estimated colorimetrically. Thirty c.c. of dilute ammonia (one of strong ammonia mixed with one of water) are added to the electrolysed solution, which is then diluted up to the 150 c.c. mark with water. It is mixed, using the spiral as stirrer, and, after standing a few minutes to allow the precipitate to settle, 100 c.c. of it are filtered off through a dry filter for the colorimetric determination. Since only two-thirds of the solution are taken for this, the quant.i.ty of copper found must be increased by one-half to get the quant.i.ty actually present.

[Ill.u.s.tration: FIG. 55.]

The ~colorimetric determination~ may be made in the manner described under that head, but where a number of a.s.says are being carried out it is more convenient to have a series of standard phials containing known amounts of copper in ammoniacal solution. By comparing the measured volume of the a.s.say solution with these, the amount of copper present is determined at a glance. These standard bottles, however, can only be economically used where a large number of a.s.says are being made daily.

A convenient plan is to get a quant.i.ty of white gla.s.s four-ounce phials, like that in fig. 55, and to label them so that they shall contain 100 c.c. when filled up to the bottom of the labels. The labels should be rendered permanent by coating with wax, and be marked with numbers indicating the milligrams of copper present. The bottles are stopped with new clean corks, and contain, in addition to the specified quant.i.ty of copper, 6 c.c. of nitric acid and 10 c.c. of strong ammonia, with sufficient water to make up the bulk to 100 c.c. The copper is best added by running in the requisite amount of a standard solution of copper, each c.c. of which contains 0.001 gram of the metal.

The standard bottles should be refilled once every three or four months, since their colorimetric value becomes slowly less on keeping. The following determinations of a set which had been in use for three months will ill.u.s.trate this. The figures indicate milligrams of copper in 100 c.c.: the first row gives the nominal and the second row the actual colorimetric value of the standards. The difference between the two shows the deterioration.

1 2 3 4 6 8 10 12 14 1 2 3 3.7 5.5 7.5 9 11 13

The amount of copper in the a.s.say is got by increasing that found colorimetrically by one-half and adding to that found on the platinum cylinder. The percentage is calculated in the usual way. The following examples will ill.u.s.trate this, as well as the method of recording the work in the laboratory book:--

--------------------------------------------- Cylinder I. + Cu 9.5410 Cylinder I. 9.5170 ------ 0.0240 By colour 100 c.c. = 0.0015} 0.0007} 0.0022 ------ ------ 0.0022 0.0262 IX. Sample. Took 5 grams.

Copper = 0.52% --------------------------------------------- Cylinder VI. + Cu 10.5705 Cylinder VI. 10.0437 ------- 0.5268 By colour, 100 c.c. = 0.0070} 0.0035} 0.0105 ------ ------ 0.0105 0.5373 Matte, No. 1070. Took 1.5 gram.

Copper = 35.82% --------------------------------------------- Cylinder XIII. + Cu 12.0352 Cylinder XIII. 11.0405 ------- 0.9947 By colour 100 c.c. = 0.0005} 0.0002} 0.0007 ------ ------ 0.0007 0.9954 X. Sample, Cake copper. Took 1.0053 gram.

Copper = 99.00% ---------------------------------------------